Dynamic Line Management (DLM) is a technique for improving the stability of DSL connections. It is particularly useful when operating DSL connections at close to their maximum speed, because under these conditions external noise affecting the transmitted signal can cause the transceivers to be unable to successfully recover the signal to be transmitted with sufficient reliability to enable the connection to be maintained. If this occurs, the connection needs to be re-established. This is referred to as a re-synch and the user notices a temporary loss of service while the connection is re-established. Re-synchs are generally found to be particularly annoying by end users.
DLM seeks to minimise such re-synchs by automatically analysing DSL connections (especially the rate of occurrence of re-synchs) and varying certain parameters which can affect the likelihood of re-synchs occurring (for example the depth of interleaving, the amount of redundancy built into the encoding used, etc.). Typically, this is done by using a number of different profiles having various different sets of values for the parameters most likely to have an impact on the stability or otherwise of a DSL connection and moving a particular connection between different profiles until a profile is found which has an acceptable stability. The profiles are applied at the local exchange (sometimes referred to—especially in the USA—as the central office) usually within a piece of equipment known as a Digital Subscriber Line Access Multiplexer (DSLAM) which houses a number of DSL transceiver units as is well known in the art.
Typically, the profiles are conceptually able to be thought of as ranging between more aggressive and less aggressive, where the more aggressive profiles tend to provide better services to the user (in terms of especially higher bit rates and lower latencies) but are more likely to result in the line being unstable, whereas less aggressive profiles tend to offer lower bit rates and/or latencies but greater stabilities.
An Alcatel Technology White Paper entitled “Dynamic Line Management for Digital Subscriber Lines” discusses DLM and suggests in overview an implementation in which there is a Validation Phase and an Operations phase. In the validation phase a connection is monitored fairly intensively to identify an appropriate profile for the line and thereafter it is monitored less intensively to ensure that the originally selected profile continues to remain valid. This paper also mentions that operators define “a set of policies for each service class”; this implies that different service classes may have different possible profiles between which they may transition. Thus, for example, for a service class associated with streaming video to an end user, the operator may enforce interleaving since there is rarely a need for a very low delay, but reliability is important to maintain a good quality picture. Furthermore, very high bandwidth is also often not required and so the operator may take away very low margin profiles as an option for such a service class (even where a line might be able to support them).
Co-pending European patent application No. 07250428.5 describes an earlier DLM solution devised by the present applicants in which very unstable data connections are detected in an efficient manner and corrective action is taken within a relatively short period of time whilst data connections which are not very unstable are monitored and transitioned between different profiles based on more thorough monitoring over a longer time-scale.
US published patent application No. 2006/0198430 describes a DLM solution in which various rules such as transition matrices are used to determine under what circumstances a transition should be made from one profile to another.
All of the DLM solutions known to the present applicants are able to set various parameters in order to try to maintain the lines at what is deemed to be an acceptable level of stability. However, the present inventors have now realised that this results in an inflexible solution which is less than optimally suited to an access network in which different DSL connections may be used for different purposes.